Most of the bacteria and archaea acquire virus resistance by integrating short viral nucleotide acid fragments into the clusters of regularly interspaced short palindromic repeats (CRISPRs). And CRISPR-based defense system can also protects them against the invading DNA and/or RNA elements. It is believed that the integrated CRISPR sequences have the ability to form a genetic memory which prevents the host from being infected. The CRISPRs and Cas (CRISPR-associated) interact and form this prokaryotic adaptive immune system.

;  A CRISPR array consists of the palindromic repeating sequences of typically 30bp that are interspaced by similar-sized acquired spacer sequences. The CRISPR loci are generally flanked bu an AT-rich leader sequence which contains promoter and the binding sites for regulatory proteins.

There three highly diverse CRISPR/Ca types exist, and major structural and functional differences are displayed in their mode of generating resistance against the invading DNA and/or RNA elements.

We focus on the type I-E CRISPR/Cas system from the model bacterium Escberichia coli k12 w3110 whose CRISPR loci has been identified so that we can download the essential sequence from NCBI(http://www.ncbi.nlm.nih.gov/nuccore/85674274?report=graph).

The mechanism of all CRISPR/Cas systems is divided into three stages: adaption, expression, and interference. At the adaption stage, the host cell acquires the resistance by integration of a new spacer sequence into a CRISPR loci. During the expression stage, cas genes are transcribed and translated. At the same time, CRISPRs are transcribed into pre-crRNAs (precursor CRISPR RNAs), and a Cas6 homolog in

Type I cleave it subsequently.

At the interference stage, the complex formed by the Cas proteins and the maturecrRNA cleave the complementary invading nucleic acids.